Digital to analog converters are widely used today in mixed-mode systems requiring monotonicity where the converter acts as an interface between the digital signal processing and analog signal processing components of such systems. Such converters are commonly used for tuning continuous time filters where a tuning algorithm is implemented in the digital domain and analog voltages based on the tuning algorithm must be obtained to tune the filter. Analog signal processing typically makes use of adaptive or programmable circuits which are tuned in the digital domain during operation to provide optimum performance.
Differential DACs are often employed in high-speed, mixed-mode systems to reduce the electronic noise produced by such systems. Specifically, a differential DAC reduces the common-mode voltage noise thereby enhancing the performance of the system. Differential DACs can be used to control equalizer filter coefficients and to provide a differential offset voltage for DC offset compensation.
Conventional resistor-string DACs of the type shown in FIG. 1 are well known in the art. DAC 10 is comprised of resistors R.sub.1 through R.sub.n which are of equal value and are coupled together in series. B.sub.[N:1] represents N digital bits, and V.sub.REFP and V.sub.REFN represent positive and negative reference voltages, respectively. DAC 10 produces a linear output as a function of the binary digital word D=[B.sub.N, B.sub.N-1 . . . B.sub.2, B.sub.1 ], which is input into DAC 10. N bits produce 2.sup.n-1 outputs. Output multiplexor 11, which is controlled by B.sub.n, controls the sign of the differential output voltage .upsilon..sub.od. Decoder block 12 is used to produce a signal to control switches S.sub.[L:0 ], where L=2.sup.N-1 -1. Exclusive OR gates 13 are used to condition bits B.sub.[N-1:1 ] so that as the counter which produces D increments or decrements, a continuous sweep of the output voltage is attained. This conditioning is achieved by decoder block 12. TABLE 1 analyzes DAC 10 when N=3 and switch S.sub.i is closed.
Conventional resistor-string DAC circuits of the type shown in FIG. 1 suffer from several drawbacks. First, the output voltage is linear and can not be shaped, thus limiting the ability to use the DAC to tune a filter. Second, the output is dependent on the value of the reference voltages V.sub.REFP and V.sub.REFN which are sometimes difficult to generate. Third, the common-mode voltage cannot be arbitrarily set, but depends on V.sub.REFP, V.sub.REFN and the resistor values.